Mesoporous aminopropyl-functionalized hybrid thin films with modulable surface and environment-responsive behavior

A first study of the behavior of amino functions in mesoporous hybrid thin films M1-x(Si(CH2)(3)NH2)(x)O2-x/2 (M = Si, Ti, Zr; 0.05 <= x <= 0.2) with accessible Im3m- or Fm3m-derived pore mesostructures is presented. An XPS study of surface nitrogen species shows two different sites corresponding to amino and ammonium groups. The ratio of these species changes with pH and is related to the nature of M, suggesting that the interaction between the organic functions and the surface M-OH groups can be tailored to tune the surface acid-base behavior. Density functional theory (DFT) calculations were used to rationalize the XPS observations showing that -NH3+ functions irreversibly transfer a proton to neighboring M-O- surface groups. The acid-base surface properties can be further modified by adding a phosphonate "capping" on the M surface sites. Our findings have a series of interesting implications in surface functionalization: attachment of biomolecules to surfaces, design of perm-selective or philicity-selective membranes, or design of catalysts that show a well-defined organic reactive function near surface hydroxyl groups.